1. Field of the Invention
The present invention relates generally to a communication apparatus and method for a communication system, and in particular, to an access communication apparatus and method.
2. Description of the Related Art
Generally, in TIA/EIA/IS-95 CDMA communication systems, a base station and a mobile station use paging channels and access channels, which are common channels, to exchange data before setting up a voice call. That is, the base station transmits a message to the mobile station over the paging channel and receives a response from the mobile station over the access channel. The mobile station transmits a message to the base station over the access channel and receives a response from the base station over the paging channel. A plurality of paging channels and the access channels may be used. The individual paging channels are identified by Walsh codes and the individual access channels are identified by long codes.
FIG. 1 illustrates a conventional procedure for exchanging messages between the base station and the mobile station over the common channels according to the prior art. Referring to FIG. 1, the base station (BS) transmits a control message to the mobile station (MS) over the paging channel, and the mobile station then transmits a response message to the base station over the access channel.
When several mobile stations, which use the same long code, simultaneously transmit messages over the access channel, a message contention occurs between the transmissions on the access channel such that the messages to be transmitted may be lost. Such an access is called “contention based random access”.
When a message contention occurs on the access channel, the mobile station retries to gain access to the base station through the access channel. In attempting to gain access, each transmission is called an “access probe”, and the access probe is comprised of a preamble and a message. The mobile stations transmit the messages over predetermined access channels, and when a message contention occurs, each mobile station detects the occurrence of the message contention after a lapse of a specific time, performs a randomized delay procedure, and then retransmits the message. Meanwhile, the mobile station initially accesses the base station using a predetermined power level, and upon failure to receive a response from the base station, the mobile station retries an access to the base station at an incrementally increased power level. In this manner, the mobile station attempts to gain access to the access channel at predetermined times. If no access is obtained in the end, this procedure is repeated again from the beginning.
Meanwhile, the mobile station determines a long code to be initially used for the common channel by performing a Hashing function on the available long code items. This long code determining method enables every mobile station to share a long code for the access channel. In addition, the message transmitted over the common channel (or access channel) can be divided into a message generated by the mobile station itself and a response message for a message transmitted from the base station. In the conventional communication system, the two messages are processed on an equal basis.
That is, in the conventional communication system, an access to the base station through the common channel is performed in a random access mode and several mobile stations simultaneously transmit the messages using the same long code, such that the messages to be transmitted may be lost due to message contention. When the message contention occurs, the mobile station accesses the access channel again after a lapse of a specific time to attempt retransmission of the message. Therefore, the base station cannot predict an access time of the access channel, thereby causing an increase in average transmission time, fluctuation of transmission time, and power dissipation of the mobile station.
Further, in the random access mode, since the mobile stations equally use the long codes for the common channel, it is not possible to separately control a probability of the message contention which occurs when each mobile station uses the access channel. In particular, a case where a mobile station initially accesses the base station and where another mobile station responds to a message from the base station are managed on an equal basis. Thus, in the latter case, when the base station is controlling an access mode of the mobile station, under conventional methods no additional advantage is gained.
In addition, a scheme is required in which the message being transmitted over the common channel has a short transmission delay time, so as to perform high-speed data communication in a situation where frequent state transitions occur between one state where the common channel is used during a packet data service and another state where the dedicated channel is used during the packet data service.
To solve this problem, Korean patent application No. 98-10617 discloses a device and method for reducing the access time and increasing the transmission efficiency when the mobile station responds to a message transmitted from the base station in a communication system supporting various services. In the disclosed device, the base station transmits a control message including channel identifier (ID) information, which can be used by the mobile station in responding to the received control message, over the common channel, and the mobile station analyzes the control message received over the common channel and then transmits a response message over a channel designated according to the channel ID information.
FIG. 2 illustrates a procedure for exchanging messages between the base station and the mobile station, using the conventional designated response mode. Referring to FIG. 2, when the base station transmits a control message with channel information, the mobile station transmits a response message over a designated channel. The designated channel can be determined either by information included in the channel designation information in the control message from the base station or by information (e.g., unique electronic serial number (ESN) of the mobile station) which is previously commonly known to the base station and the mobile station. Here, the channel is identified by the long code.
FIG. 3 illustrates a forward link control message transmitted from the base station to the mobile station according to the prior art. The control message includes information about whether the channel is designated or not, channel number and data rate as well as the contents of the control message. Here, the channel number indicates a reverse common channel number (or long code number) and a corresponding power control channel number. When a unique long code of the mobile station is used for the reverse common channel, the channel number is used only for the power control channel number.
In this method, however, the base station must know a transmission start time point where the mobile station transmits a response message for the control message, and to do so, the base station must transmit separate time information to the mobile station. More specifically, the mobile station can start transmission of a preamble and a message at a 1.25 ms boundary based on the system time. Here, the term “1.25 ms boundary” is equivalent to another term “slot boundary” or “start point of slots”. Therefore, it is difficult for the base station to detect an exact transmission start time point out of the transmission start time points of the mobile station, which are available every 1.25 ms. Therefore, it is preferable to use a method for designating a transmission start time point of the reverse message for the mobile station. However, the time information generally requires many bits, making it inefficient to include the time information in the control message.